Peter, you puzzle over the difference between a software interrupt and calling a subroutine. An example: I once wrote a program with a veeeery long workloop and some heavy interrupt activity. In order to a) catch every serial byte and b) process the record received on the serial bus to get the turnaround of the port sooner I made the serial interrupt activate a software interrupt of the lowest priority of any interrupt. This software interrupt handles the processing of the record received. This allowed pushing the serial interrupt up in priority while not making it 'kill' other interrupts because of the lenghty record processing.